- Email: [email protected]
A model of material flow during friction stir welding
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
A model of material flow during friction stir welding Carter Hamilton a,⁎, Stanisław Dymek b , Marek Blicharski b a
Miami University, Department Of Mechanical and Manufacturing Engineering, Kreger Hall, Oxford, OH 45056, USA AGH University Of Science And Technology, Faculty of Metals Engineering and Industrial Computer Science, 30-059 Kraków, Al. Mickiewicza 30, Poland
b
AR TIC LE D ATA
ABSTR ACT
Article history:
Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld
Received 1 March 2007
configuration. A banded microstructure of interleaved layers of particle-rich and particle-
Received in revised form
poor material comprised the weld nugget. Scanning and transmission electron microscopy
26 August 2007
revealed the strong presence of tin within the particle-rich bands, but TEM foils taken from
Accepted 1 October 2007
the TMAZ, HAZ and base material showed no indication of Sn-containing phases. Since tin is limited to the surface of the pre-weld extrusions, surface material flowed into the nugget
Keywords:
region, forming the particle-rich bands. Similarly, the particle-poor bands with no tin
Friction stir welding
originated from within the thickness of the extrusions. A model of material flow during
Aluminum
friction stir welding is proposed for which the weld nugget forms as surface material
Microstructure
extrudes from the retreating side into a plasticized zone surrounding the FSW pin. The
Weld nugget
extruded column buckles between the extrusion force driving the material into the zone and
Material flow
the drag force of the in-situ material resisting its entry. A banded microstructure of interleaved surface material and in-situ material, therefore, develops. The model successfully describes several of the experimentally observed weld characteristics, but the model is limited to specific conditions of material flow and assumptions regarding steadystate. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
Invented in 1991 by The Welding Institute, Friction Stir Welding (FSW) is a novel solid-state joining process that is gaining popularity in the manufacturing sector [1,2]. FSW utilizes a rotating tool design to induce plastic flow in the base metals and essentially “stirs” the pieces together. During the welding process, a pin, attached to the primary tool, is inserted into the joint with the shoulder of the rotating tool abutting the base metals. As the tool traverses the joint, the rotation of the shoulder under the influence of an applied load heats the metal surrounding the joint and with the rotating action of the pin induces metal from each workpiece to flow and form the weld. The microstructure resulting from the influence of plastic deformation and elevated temperature is characterized by a central weld nugget surrounded by a thermo-mechani-
cally affected zone (TMAZ) and heat affected zone (HAZ). The welded joint is fundamentally defect-free and displays excellent mechanical properties when compared to conventional fusion welds [3–6]. Over the last fifteen years, numerous investigations have sought to characterize the principles of FSW and to model the microstructural evolution. The current status of FSW research has been well summarized by Mishra and Ma [7]. The flow of material during FSW is a complex process that is not fully understood despite numerous investigations and models. Several studies have compared material flow during FSW with wrought metal processes and have modeled weld nugget development as an extrusion process [7,8]. In particular, Krishnan [9] and Sutton et al. [10] hypothesized that the nugget forms as a volume of material from the weld surface extrudes into the joint during each revolution of the tool.
⁎ Corresponding author. Tel.: +1 513 529 0722; fax: +1 513 529 0717. E-mail address: [email protected] (C. Hamilton). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.10.002
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Seidel and Reynolds utilized fluid mechanics to create a twodimensional model of FSW as a non-Newtonian fluid flowing around a rotating cylinder [11]. Though each of these models predict some characteristics of the weld nugget, there are limitations. For example, the models do not adequately describe the formation of the banded microstructure of particle-rich and particle-poor regions commonly observed during FSW [9,12–14]. The following study investigates friction stir weld nugget development during the welding of 6061-T6 extrusions that were plated with tin to facilitate the tracking of material flow. A model is proposed in which surface material from the retreating side extrudes into the plasticized material zone within the weld thickness. As the extrusion enters the plasticized region, the column buckles under the drag force, resulting in an interleaved structure of surface material and in-situ material. The model further predicts that the extrusion column exhibits a higher particle density than the plasticized zone, thus leading to the non-uniform particle distribution in the weld nugget.
2.
Experimental Procedure
Aluminum 6061-T6 extrusions produced in accordance with ASTM B 317 with a thickness of 6.35 mm and a width of 195.0 mm were obtained and welded in the configuration represented in Fig. 1. As shown in the diagram, the longitudinal grain directions of the individual extrusions are welded perpendicular to one another, such that FSW occurs along the L-direction of the advancing side and along the LT-direction of the retreating side. This configuration was selected in cooperation with the welder, whose FSW tooling produces electrical busbar-corners at 90° angles. As shown in Fig. 2, with a clockwise tool rotation FSW occurs along the L-direction of the advancing side (rotation of the tool is in the same direction as the weld direction) and along the LT-direction of the retreating side (rotation of the tool is in the opposite direction of the weld direction). Though most FSW studies utilize conventional butt-welds for which welding occurs along the longitudinal direction of both workpieces, the material flow characteristics of the 90° butt-weld are expected to be the same as those of a conventional butt-weld due to the extensive plastic deformation and elevated temperature experienced during FSW. Prior to welding, the extrusions were plated with tin, 0.05 mm thick, in order to trace the flow of surface material during the process. The extrusion edges were not masked during plating; therefore, the long edge of the advancing side does have some tin coverage, but since the extrusions were
Fig. 2 – TEM of Mg2Si strengthening phase in the 6061 alloy (unaffected material) in the form of coherent rods.
cut to length after plating, the short edge of the retreating side is without tin. The diameter of the FSW tool shoulder was 21.0 mm, the pin diameter was 6.5 mm and the pin depth was 5.5 mm. More specific details of the tool design are proprietary to the welding company, but Mishra and Ma [7] and other studies [15,16] have reviewed many of the common FSW tool designs that are also indicative of those utilized in this investigation. The extrusions were friction stir welded with a tool rotation speed of 900 rpm, a traverse speed in the weld direction of 5.0 mm/s and an applied force of 22.5 kN. The microstructural work was performed on the plane perpendicular to the weld direction. A 50 mm wide crosssection centered on the weld was ground on sand papers to remove any residual damage from the cutting process. For optical microscopy the samples were initially electropolished in a solution of perchloric acid and ethanol (1:5) at 10 V and 12 °C for one minute and then chemically etched in a solution of 5 ml HF, 10 ml H2SO4 and 85 ml water. The light microscopy examination was performed on a Leica DMLM microscope and the scanning electron microscopy (SEM) on a Hitachi S-3500 N equipped with EDS Noran 986B-1SPS. The more demanding requirements of transmission electron microscopy (TEM) required more precise specimen preparation. First, a 0.4 mm thick slice was cut off from the etched metallographic sample. The slice was further mounted on a steel block (non-etched side up) and thinned down to approximately 0.08 mm. After detaching from the block, 3 mm disks were excised from particular regions: center of the nugget, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and unaffected material. Before electropolishing, the disks were dimpled to approximately 0.02 mm depth to ensure that a perforation during electropolishing appeared in the disk centers. The electropolishing was performed in 30% nitric acid/methanol solution at 30 V and −30 °C on a Struers Tenupol-3 jet electropolisher. TEM investigations were performed on a JEOL 2010 ARP equipped with EDS Inca Oxford and camera CCD Orius Gatan.
3. Fig. 1 – Schematic of corner weld geometry and FSW orientation.
1207
Results and Discussion
The investigated 6061 alloy belongs to the age-hardenable group of aluminum alloys. Its microstructure well away from
1208
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Fig. 3 – Representative optical micrograph of friction stir weld nugget; upper edge corresponds to the top surface of joined extrusions; dark field image.
the weld is composed of equiaxed grains (approximately 50 μm in diameter) with the dispersed strengthening phase, Mg2Si, in the form of coherent rods, as shown in Fig. 2. Fig. 3 shows the microstructure of a typical FSW nugget produced during this study. The complex flow pattern of the FSW process is clearly evident, and the weld nugget is primarily characterized by the banded appearance, often referred to as the “onion ring” structure [9,14]. The contrast between the banded layers results from an uneven distribution of secondary phase particles. The lighter bands in the optical image reflect a higher density of secondary phase particles, and the darker bands reflect a lower one. Since the visual appearance of the bands depends on the instrumentation used, it is more appropriate to differentiate the bands as “particle-rich” and “particle-poor”, terminology consistent with that introduced by Sutton et al. who observed the same phenomenon in FSW 2024-T351 rolled sheet [10]. The scanning electron images in Fig. 4 taken near the nugget/TMAZ interface even more clearly demonstrate the non-uniform distribution of the secondary phase particles in the banded microstructure. Fig. 4a clearly shows the interleaving of particle-rich and particle-poor layers that develop during FSW. Even more striking, however, is Fig. 4b, which shows a very distinct boundary between a particle-rich region of the weld nugget (right) and the particledeficient TMAZ (left). Some researchers have concluded that the weld nugget structure is actually an interleaving of layers of fine-equiaxed, recrystallized grains with coarse recrystallized grains [17,18]. During this investigation, the grains within the nugget showed little deformation in the flow direction, suggesting that recrystallization did, indeed, occur. The grain size, however, in both the particle-rich and particle-poor bands was equivalent, approximately 10 μm to 20 μm [12]. Fig. 5a is a TEM micrograph of a foil taken from a region near the center of the weld nugget. The secondary phase particles of the particle-rich bands are clearly evident, and chemical analysis by EDS of these phases indicates a strong tin (Sn) content. The particle rich band is primarily composed of the Mg2Si and Mg2Sn phases [19]. Fig. 5b is of a foil taken from the TMAZ, the region adjacent to the weld nugget. The micrograph reveals a high dislocation density in this plastically deformed area, but neither Sn particles nor any strengthening phases are present. Also, foils excised from within the HAZ and base material do not indicate the presence of any Sn-containing phases.
Since tin is only present on the surface of the extrusions as plating, the unique occurrence of tin in the weld nugget is extremely significant. Material comprising the weld nugget, therefore, must originate from the extrusion surface since this is the only viable source of tin. Similarly, the material within the TMAZ and HAZ zones must originate from within the extrusion thickness since TEM demonstrated the absence of tin from these regions. The tin-rich surface and the tindeficient thickness effectively act as tracers of material flow during welding and the evolution of the weld nugget. The TEM results demonstrate that the particle-rich bands containing tin must be comprised of surface material that has flowed into the region that ultimately transforms into the weld nugget. Similarly, since the particle-poor bands do not contain discernable amounts of tin, the material comprising this structure must originate from within the thickness of the extrusions. The banded microstructure of the weld nugget, therefore, is composed of alternating layers of surface material rich in Sn-containing particles and plasticized material already present in the extrusion thickness, i.e. insitu material. Fig. 3 clearly highlights the banded structure of the weld nugget, but it also reveals a ligament, or “tail”, of the weld on the retreating side extending from the primary nugget body to the extrusion surface. The nugget and tail morphology suggests that surface material is introduced from the retreating side into the plasticized region surrounding the FSW pin. As indicated in Fig. 3, the average distance from the tail/ surface intersection to the weld centerline, i.e. the tool center, is 9.0 mm.
Fig. 4 – BSE (a) and SE (b) SEM images near friction stir nugget/ TMAZ interface.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1209
Fig. 5 – TEM micrograph of: a) weld nugget center revealing Sn-rich particles (with EDS spectrum), b) TMAZ revealing no Sn.
These observations are consistent with those made by Colligan in his study of material flow during the friction stir welding of 6061-T6 using steel shot tracers and a “stop action” technique [20]. In examining the flow of material around the FSW keyhole, Colligan noted that below the weld surface, where the steel shot was chaotically distributed, the steel shot was deposited as a continuous line behind the welding pin, akin to the particle-rich bands observed this study. Colligan concluded that surface material was extruded from the retreating side of the pin and deposited in the wake of the tool, in agreement with the tail/nugget morphology of this investigation. Guerra et al. in their study of FSW 6061 with a faying surface tracer and frozen pin technique similarly concluded that material from the front of the retreating side of the pin extrudes between deformed surface material
rotating with the tool and parent material into the area behind the pin [21]. The model of FSW as an extrusion process is further substantiated by Reynolds who used a marker insert technique to study material flow in the welding of 2195-T8 [22]. Reynolds concluded that FSW is essentially an in-situ extrusion process where the tool shoulder, pin, backing plate and base metal effectively form an “extrusion chamber”. Reynolds, however, did not specifically observe extrusion of surface material from the retreating side of the weld. Rather, the tracers in this study revealed a downward motion of surface material on the advancing side coupled with the vertical rise of material from the bottom surface of the retreating side within the pin diameter. Fig. 6 shows a macroscopic plan view of the friction stir welded extrusions. Highlighted in the figure is the flash of
1210
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Fig. 7 – Schematic representation of plasticized region during friction stir welding. Fig. 6 – Plan view of friction stir welds showing material flash on retreating side.
material consistently present along the length of the weld on the retreating side. The average distance of the flash from the weld centerline is 6.8 mm, a distance that corresponds with the location of the tail/surface intersection of the nugget profile. The presence of the flash and nugget tail on the retreating side identifies this location as the potential source of surface material within the weld nugget. Based upon these results, a model of material flow is proposed for which surface material on the retreating side is extruded into the plasticized zone at a specific radial distance from the tool center, re. The extruding surface material interacts with the in-situ material to form the banded structure of the weld nugget. This model is developed in the proceeding sections.
4.
Proposed Model of Material Flow
4.1.
Conditions for Extrusion
Consider the cross-sectional profile of the FSW process for a typical butt weld configuration. As the tool and pin rotate, the flow stress in a region surrounding the weld joint is exceeded, facilitating solid-state material flow. The shoulder of the rotating tool plasticizes a thin, cylindrical volume of material on the surface, and the pin plasticizes a volume of material within the joint thickness. The net effect is a flow-capable region with a cross-sectional shape resembling that of the inverted bell shown in Fig. 7. On the retreating side of the weld, the rotation of the tool opposes the traversing weld direction. Due to the frictional forces with the tool, the shoulder displaces surface material in both the direction of rotation and in the weld direction. On the retreating side, therefore, surface material rotating with the tool shoulder is forced to flow under material moving in the weld direction as depicted in Fig. 8. In essence, the FSW tool shoulder extrudes a volume of surface material from the retreating side into the plasticized region of the joint. The extrusion process does not occur at
every point along tool radius of the retreating side, but occurs at the radius where the velocity of material following tool rotation is equal and opposite to the velocity of material following the tool traverse. Heurtier et al. identified and modeled three types of motion during friction stir welding, circumventing, torsional and vortex [23]. The circumventing and torsional motions are associated with the flow of surface material, and vortex motion is associated with the flow of thickness material due to the action of the tool pin. Circumventing motion, therefore, is the motion of surface material around the tool shoulder, and torsional motion is the rotational motion of surface material within the interaction layer under the tool shoulder. Heurtier et al. demonstrated that the circumventing velocity, ucir, in the weld direction is given by: cos2h ucir ¼ vw 1 R2 2 r
ð1Þ
where vw is the traversing weld velocity, R is the radius of the tool shoulder, r is the radial distance from the tool center and θ is the radial angle relative to the weld centerline. The torsional velocity along the weld direction, utor, is given by: z utor ¼ r vtor sinh t
ð2Þ
where vtor is the rotational velocity of the material under the tool shoulder, z is the depth into the material, t is the
Fig. 8 – Material flow pattern on retreating side of the weld.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1211
thickness of the interaction layer and all other terms have their previous meaning. Extrusion into the weld nugget then occurs at the location, re, on the retreating side where ucir is equal to utor, such that: cos2h z ¼ re vtor sinh vw 1 R2 2 re t
ð3Þ
Assuming that extrusion occurs at the depth of the interaction layer (z = t) and at a radial angle of − 90° on the retreating side relative to the weld centerline, then Eq. (3) reduces to: re ¼
vw R2 1þ 2 vtor re
ð4Þ
vtor depends on the angular velocity of the tool shoulder, ω, but is certainly lower as the material flow will naturally lag behind the shoulder rotation due to the inherent viscosity. Introducing a scaling factor, β, to account for the lag, the relationship between vtor and ω at the extrusion distance becomes: vtor ¼ b x re
ð5Þ
Combining Eqs. (4) and (5) yields the following expression for the scaling factor:
Fig. 10 – Extrusion ring in the interaction layer with the tool shoulder.
during a single rotation of the tool; therefore, the traverse distance is given by the expression: d¼
2p vw x
ð7Þ
Based on the welding parameters utilized in this investigation (R = 10.5 mm, vw = 5 mm/s, ω = 94.2 s− 1) and setting re to the experimentally measured value of 6.8 mm for the average distance of the flash from the weld centerline, Eq. (6) yields a scaling factor of 0.004.
where all terms have their previous meaning. Surface material from the retreating side is then extruded during each tool revolution through the area shown in Fig. 9. The extrusion area that is swept per tool revolution can be effectively considered the die opening into the plasticized region of the joint thickness. If it is further assumed that opening is rectangular, then the die opening area, Adie, is given by:
4.2.
Adie ¼
b¼
vw R2 1 þ x r2e r2e
ð6Þ
Development of the Extrusion Column
In order to determine the volume of material extruded into the plasticized region, first consider the amount of tool advance in the weld direction per revolution as depicted in Fig. 9. As the tool rotates and traverses in the weld direction, a point initially located at A will displace a distance, d, to point B
2p vw w x
ð8Þ
where w is the width of the die opening. As previously mentioned, the tool shoulder plasticizes a thin, cylindrical volume of material at the weld surface, and an interaction layer of thickness, t, is formed. The material extruded on the retreating side, therefore, originates from the interaction layer and is defined by the volume of material contained within a ring centered on the tool at the extrusion distance, re, as illustrated in Fig. 10. If the width of the extrusion ring is equal to the width of the die opening, w, then the volume of the ring per revolution, Vring, is given by: Vring ¼ p ðre þ wÞ2 t p r2e t
ð9Þ
Simplifying the above equation and removing all second order terms in w yields: Vring ¼ 2p re wt
Fig. 9 – Tool advance and extrusion area per revolution.
ð10Þ
The material extruded into the plasticized region has a rectangular cross-section and a length, l, akin to the production of bar stock during conventional extrusion processes. Assuming that the entire volume of the material ring is extruded into the plasticized region for each revolution of the tool, then the length of the extruded column is found by
1212
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
equating Eq. (10) with the volume of the extrusion, itself, such that: 2p re wt ¼ Adie l
ð11Þ
Substituting Eq. (8) into Eq. (11) and solving for l yields the following expression for the extrusion length: l¼
re tx vw
ð12Þ
Surface material is extruded into the plasticized region under the extrusion force applied by the FSW tool; however, the extrusion is resisted by the drag force of the plasticized material already present in the area. Under the action of these two forces, the extruded column of surface material buckles as represented in Fig. 11. As the tool continues to rotate and traverse, more material is extruded into the region, forcing the buckled, extruded columns further into the plasticized zone and introducing new material into the region. Ultimately, a quasi steady-state is reached such that the region contains bands of extruded surface material interleaved with bands of in-situ thickness material, as observed in Fig. 3. Experimentally determining the amount of material extruded into the plasticized region begins by defining the thickness of the interaction layer, t, as the depth (or thickness) of the nugget tail at the radial extrusion distance. With this definition, the average thickness of the interaction layer for these welding conditions is 0.1 mm. Utilizing this value to evaluate Eq. (12) and using the β-factor of 0.004, the length of the material extruded per tool revolution then becomes 12.6 mm. The microstructure shown in Fig. 3, however, represents quasi steady-state conditions. Assuming that steady-state is achieved within one second, i.e. fifteen tool revolutions, then the length of material extruded per second into the plasticized region is 189 mm. This value predicted by the proposed model can be verified against the experimental data by determining the area fraction occupied by the buckled columns within the weld nugget. The cross-sectional area of the weld nugget in Fig. 3 was estimated as 45 mm2 and that of the buckled, extruded
Fig. 12 – Distribution of particles due to centrifugal force of tool rotation.
material as 35 mm2. The average width of an extruded column within nugget tail is 0.20 mm (a value near the magnitude of the tool advance per revolution, 0.33 mm); therefore, the length of extruded material present in the weld nugget is approximately 175 mm. The experimentally determined value is in good agreement with that predicted by the model (8% difference). Discrepancy between the two values centers on the establishment of steady-state conditions. Though the assumption that quasi steady-state is achieved after one second is reasonable, it is, however, somewhat arbitrary. The model's prediction of the extrusion length is sensitive to the input value of the time required to reach steady-state. For example, if steady-state conditions are established within 0.9 seconds, then the model yields an extrusion length of 170 mm. In the same way, it should be noted that the experimentally determined value of the extrusion length is sensitive to the average width of the extrusion column, which is not easily or accurately measured from the micrographs. Though examination of the weld nugget and the development of the extrusion column validate the proposed model under specific conditions and measurements, it also underscores the difficulty in defining steady-state conditions during FSW and in utilizing average properties to characterize such a complex structure.
4.3.
Fig. 11 – Buckling of extruded surface material in plasticized zone.
Development of the Non-Uniform Particle Distribution
The microstructures of the alloys of interest contain numerous secondary phase particles that are generally coarse and have particle diameters in excess of 5 μm. Within the interaction layer between the tool shoulder and the weld surface, particles will experience a centrifugal force due to the tool rotation that directs them toward the edge of the tool as depicted in Fig. 12. The centrifugal force, however, is resisted by the drag force within the plasticized material. A particle under the action of a centrifugal force, Fcent, will motion until the opposing force, Fdrag, is equal in magnitude. As a result of the tool rotation, larger, heavier particles will be forced to greater distances/radii from the weld joint than smaller, lighter particles. The relationship between the particle radius,
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1213
Fig. 15 – Build-up of particle rich material at the extrusion location.
Fig. 13 – Microstructure around FSW pin revealing particle banding ~1 mm below surface.
a, and the radial distance from the tool center, r, can be developed by equating the two forces acting on a particle: Fcent ¼ Fdrag
ð13Þ
mv2 1 ¼ qAl v2 ACd 2 r
ð14Þ
Eq. (14) assumes that the plasticized interaction layer is adequately modeled as a fluid exerting drag on a mass in motion; therefore, m is the particle mass, ρAl is the density of aluminum, v is the particle velocity, A is the particle crosssectional area and Cd is the drag coefficient. If it is further assumed that each particle is spherical in shape and has a uniform density of ρpart, then Eq. (14) then reduces to the following expression: r¼
8 qpart 1 a 3 qAl Cd
ð15Þ
Since all terms other than r and a are constant, Eq. (15) can be rewritten as: r ¼ aa
ð16Þ
where α is a constant, and the linear relationship between the size of a particle and the radial distance to which the particle will travel during tool rotation is revealed. Thus, at the
extrusion distance, the surface material will have a higher particle density than both the material located close to the tool center and the plasticized material in the extrusion thickness. Material flowing into the weld nugget from the retreating side, therefore, will have a higher particle density than the in-situ material, creating the non-uniform particle distribution within the nugget. Fig. 13 is an optical micrograph taken approximately 1 mm below the weld surface around the key hole. The characteristic banded microstructure is evident on the surface around the hole, but there is no clear data to support a predictable particle distribution on the weld surface as indicated by Eq. (16). These Sn-rich bands on the surface arise from the plating and do indicate that the tool rotation accumulates Sn-containing particles that then extrude into the nugget zone, forming the characteristic interleaved layers of the weld nugget. The distribution of particles on the weld surface also indicates that the material extruded into the plasticized region at the extrusion distance, re, will have a higher particle density than the material already present in the plasticized zone. As the particle-rich extrusions buckle and continue to press into the region, the steady-state microstructure will contain high particle density bands interleaved with low particle density bands as schematically shown in Fig. 14. In addition, as material enters the plasticized zone, the extrusions will experience a frictional force between the boundary of plasticized material and the “unaffected” material, similar to the interaction between an extrusion billet and the press container during conventional extrusion processes, as represented in Fig. 15 and as experimentally observed in Fig. 3 in the tail section of the weld nugget. The frictional forces with the “unaffected” material cause a reverse material flow, resulting in a build-up of surface material within the plasticized zone at the extrusion distance. Since the extrusion is rich in secondary phase particles, the build-up is primarily characterized by a high particle density.
5.
Fig. 14 – Extrusion of high particle density material in plasticized zone.
Conclusions
Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld configuration. The weld nugget exhibited a banded microstructure consisting of alternating layers of material “rich” in secondary phase particles and material “poor” in the particles. Transmission electron microscopy revealed that the particle-rich bands contained a
1214
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
strong presence of tin, while TEM foils taken from the TMAZ, HAZ and base material thickness did not indicate any Sncontaining phases. Since tin is only present on the surface of the extrusions, the particle-rich bands represent surface material that has flowed into the nugget region. The particle-poor bands with no tin, therefore, must be material that originates from within the thickness of the extrusions. A model of material flow during friction stir welding was proposed. Under this model, the weld nugget forms as surface material is extruded from the retreating side into the region of plasticized material around the FSW pin and under the tool shoulder. As the surface material extrudes into the region, the extrusion column buckles under the influence of the extrusion force driving the material into the area and the drag force of the in-situ material resisting the entry. As a result, a banded microstructure of interleaved surface material and in-situ material develops. The model successfully depicts many of the weld characteristics experimentally observed; however, the efficacy of the model is currently limited to specific conditions and assumptions regarding steady-state material flow.
Acknowledgements The authors would like to specifically thank the Philip and Elaina Hampton Fund for Faculty International Initiatives and the Polish Ministry of Science and Higher Education (Grant No. N 507 094 32/2648) for making this research collaboration a reality.
REFERENCES [1] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, G.B. Patent Application No. 9125978.8 (December 1991). [2] Dawes C, Thomas W. TWI Bulletin November/December 1995;6:124. [3] Sutton MA, Yang B, Reynolds AP, Yan J. Mat Sci Eng A 2004;364:66.
[4] Dracup BJ, Arbegast WJ. Proceedings of the 1999 SAE Aerospace Automated Fastening Conference & Exposition, Memphis, TN; October 5–7, 1999. [5] von Strombeck A, dos Santos JF, Torster F, Laureano P, Kocak M. Proceedings of the First International Symposium on Friction Stir Welding. CA, USA: Thousand Oaks; June 14–16, 1999. [6] Hori H, Tanikawa H, Seo N, Namba K. Proceedings of the 6th International Symposium on Friction Stir Welding. Canada: Montreal; October 10 – 13, 2007. [7] Mishra RS, Ma ZY. Mater Sci Eng R Rep 2005;50:1. [8] Arbegast WJ. In: Jin Z, Beaudoin A, Bieler TA, Radhakrishnan B, editors. Hot deformation of aluminum alloys III. Warrendale, PA, USA: TMS; 2003. p. 313. [9] Krishnan KN. Mat Sci Eng A 2002;327:246. [10] Sutton MA, Yang B, Reynolds AP, Taylor R. Mater Sci Eng A 2002;323:160. [11] Seidel TU, Reynolds AP. Sci Technol Weld Join 2003;3(8):175. [12] Hamilton BC, Dymek S, Blicharski M. Arch Metall Mater 2007;52:66. [13] Hamilton C, Dymek S, Blicharski M. Proceedings of the 2007 TMS Annual meeting and exposition, Orlando, FL; February 25 – March 1, 2007. [14] Attallah MM, Davis CL, Strangwood M. proceedings of the 6th International Symposium on Friction Stir Welding. Canada: Montreal; October 10 – 13, 2007. (on CD-ROM). [15] Thomas WM, Nicholas ED, Smith SD. In: Das SK, Kaufman JG, Lienert TJ, editors. Aluminum 2001 – Proceedings of the TMS 2001 Aluminum automotive and joining sessions. TMS; 2001. p. 213. [16] Thomas WM, Johnson KI, Wiesner CS. Adv Eng Mater 2003;5:485. [17] Mahoney MW, Rhodes CG, Flintoff JG, Spurling RA, Bingel WH. Metall Mater Trans A 1998;29:1955. [18] Sato YS, Park SHC, Kokawa H. Metall Mater Trans A 2001;32:3023. [19] Jung IH, Kang DH, Park WJ, Kim NJ, Ahn S. Calphad 2007;31:192. [20] Colligan K. Weld J 1999;78(7):229S. [21] Guerra A, Schmidt C, McClure JC, Murr LE, Nunes AC. Mat Char 2002;49(2):95. [22] Reynolds AP. Sci Technol Weld Join 2000;5(2):120. [23] Heurtier P, Jones MJ, Desrayaud C, Driver JH, Montheillet F, Allehaux D. J Mater Process Technol 2006;171:348.
A model of material flow during friction stir welding Carter Hamilton a,⁎, Stanisław Dymek b , Marek Blicharski b a
Miami University, Department Of Mechanical and Manufacturing Engineering, Kreger Hall, Oxford, OH 45056, USA AGH University Of Science And Technology, Faculty of Metals Engineering and Industrial Computer Science, 30-059 Kraków, Al. Mickiewicza 30, Poland
b
AR TIC LE D ATA
ABSTR ACT
Article history:
Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld
Received 1 March 2007
configuration. A banded microstructure of interleaved layers of particle-rich and particle-
Received in revised form
poor material comprised the weld nugget. Scanning and transmission electron microscopy
26 August 2007
revealed the strong presence of tin within the particle-rich bands, but TEM foils taken from
Accepted 1 October 2007
the TMAZ, HAZ and base material showed no indication of Sn-containing phases. Since tin is limited to the surface of the pre-weld extrusions, surface material flowed into the nugget
Keywords:
region, forming the particle-rich bands. Similarly, the particle-poor bands with no tin
Friction stir welding
originated from within the thickness of the extrusions. A model of material flow during
Aluminum
friction stir welding is proposed for which the weld nugget forms as surface material
Microstructure
extrudes from the retreating side into a plasticized zone surrounding the FSW pin. The
Weld nugget
extruded column buckles between the extrusion force driving the material into the zone and
Material flow
the drag force of the in-situ material resisting its entry. A banded microstructure of interleaved surface material and in-situ material, therefore, develops. The model successfully describes several of the experimentally observed weld characteristics, but the model is limited to specific conditions of material flow and assumptions regarding steadystate. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
Invented in 1991 by The Welding Institute, Friction Stir Welding (FSW) is a novel solid-state joining process that is gaining popularity in the manufacturing sector [1,2]. FSW utilizes a rotating tool design to induce plastic flow in the base metals and essentially “stirs” the pieces together. During the welding process, a pin, attached to the primary tool, is inserted into the joint with the shoulder of the rotating tool abutting the base metals. As the tool traverses the joint, the rotation of the shoulder under the influence of an applied load heats the metal surrounding the joint and with the rotating action of the pin induces metal from each workpiece to flow and form the weld. The microstructure resulting from the influence of plastic deformation and elevated temperature is characterized by a central weld nugget surrounded by a thermo-mechani-
cally affected zone (TMAZ) and heat affected zone (HAZ). The welded joint is fundamentally defect-free and displays excellent mechanical properties when compared to conventional fusion welds [3–6]. Over the last fifteen years, numerous investigations have sought to characterize the principles of FSW and to model the microstructural evolution. The current status of FSW research has been well summarized by Mishra and Ma [7]. The flow of material during FSW is a complex process that is not fully understood despite numerous investigations and models. Several studies have compared material flow during FSW with wrought metal processes and have modeled weld nugget development as an extrusion process [7,8]. In particular, Krishnan [9] and Sutton et al. [10] hypothesized that the nugget forms as a volume of material from the weld surface extrudes into the joint during each revolution of the tool.
⁎ Corresponding author. Tel.: +1 513 529 0722; fax: +1 513 529 0717. E-mail address: [email protected] (C. Hamilton). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.10.002
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Seidel and Reynolds utilized fluid mechanics to create a twodimensional model of FSW as a non-Newtonian fluid flowing around a rotating cylinder [11]. Though each of these models predict some characteristics of the weld nugget, there are limitations. For example, the models do not adequately describe the formation of the banded microstructure of particle-rich and particle-poor regions commonly observed during FSW [9,12–14]. The following study investigates friction stir weld nugget development during the welding of 6061-T6 extrusions that were plated with tin to facilitate the tracking of material flow. A model is proposed in which surface material from the retreating side extrudes into the plasticized material zone within the weld thickness. As the extrusion enters the plasticized region, the column buckles under the drag force, resulting in an interleaved structure of surface material and in-situ material. The model further predicts that the extrusion column exhibits a higher particle density than the plasticized zone, thus leading to the non-uniform particle distribution in the weld nugget.
2.
Experimental Procedure
Aluminum 6061-T6 extrusions produced in accordance with ASTM B 317 with a thickness of 6.35 mm and a width of 195.0 mm were obtained and welded in the configuration represented in Fig. 1. As shown in the diagram, the longitudinal grain directions of the individual extrusions are welded perpendicular to one another, such that FSW occurs along the L-direction of the advancing side and along the LT-direction of the retreating side. This configuration was selected in cooperation with the welder, whose FSW tooling produces electrical busbar-corners at 90° angles. As shown in Fig. 2, with a clockwise tool rotation FSW occurs along the L-direction of the advancing side (rotation of the tool is in the same direction as the weld direction) and along the LT-direction of the retreating side (rotation of the tool is in the opposite direction of the weld direction). Though most FSW studies utilize conventional butt-welds for which welding occurs along the longitudinal direction of both workpieces, the material flow characteristics of the 90° butt-weld are expected to be the same as those of a conventional butt-weld due to the extensive plastic deformation and elevated temperature experienced during FSW. Prior to welding, the extrusions were plated with tin, 0.05 mm thick, in order to trace the flow of surface material during the process. The extrusion edges were not masked during plating; therefore, the long edge of the advancing side does have some tin coverage, but since the extrusions were
Fig. 2 – TEM of Mg2Si strengthening phase in the 6061 alloy (unaffected material) in the form of coherent rods.
cut to length after plating, the short edge of the retreating side is without tin. The diameter of the FSW tool shoulder was 21.0 mm, the pin diameter was 6.5 mm and the pin depth was 5.5 mm. More specific details of the tool design are proprietary to the welding company, but Mishra and Ma [7] and other studies [15,16] have reviewed many of the common FSW tool designs that are also indicative of those utilized in this investigation. The extrusions were friction stir welded with a tool rotation speed of 900 rpm, a traverse speed in the weld direction of 5.0 mm/s and an applied force of 22.5 kN. The microstructural work was performed on the plane perpendicular to the weld direction. A 50 mm wide crosssection centered on the weld was ground on sand papers to remove any residual damage from the cutting process. For optical microscopy the samples were initially electropolished in a solution of perchloric acid and ethanol (1:5) at 10 V and 12 °C for one minute and then chemically etched in a solution of 5 ml HF, 10 ml H2SO4 and 85 ml water. The light microscopy examination was performed on a Leica DMLM microscope and the scanning electron microscopy (SEM) on a Hitachi S-3500 N equipped with EDS Noran 986B-1SPS. The more demanding requirements of transmission electron microscopy (TEM) required more precise specimen preparation. First, a 0.4 mm thick slice was cut off from the etched metallographic sample. The slice was further mounted on a steel block (non-etched side up) and thinned down to approximately 0.08 mm. After detaching from the block, 3 mm disks were excised from particular regions: center of the nugget, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and unaffected material. Before electropolishing, the disks were dimpled to approximately 0.02 mm depth to ensure that a perforation during electropolishing appeared in the disk centers. The electropolishing was performed in 30% nitric acid/methanol solution at 30 V and −30 °C on a Struers Tenupol-3 jet electropolisher. TEM investigations were performed on a JEOL 2010 ARP equipped with EDS Inca Oxford and camera CCD Orius Gatan.
3. Fig. 1 – Schematic of corner weld geometry and FSW orientation.
1207
Results and Discussion
The investigated 6061 alloy belongs to the age-hardenable group of aluminum alloys. Its microstructure well away from
1208
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Fig. 3 – Representative optical micrograph of friction stir weld nugget; upper edge corresponds to the top surface of joined extrusions; dark field image.
the weld is composed of equiaxed grains (approximately 50 μm in diameter) with the dispersed strengthening phase, Mg2Si, in the form of coherent rods, as shown in Fig. 2. Fig. 3 shows the microstructure of a typical FSW nugget produced during this study. The complex flow pattern of the FSW process is clearly evident, and the weld nugget is primarily characterized by the banded appearance, often referred to as the “onion ring” structure [9,14]. The contrast between the banded layers results from an uneven distribution of secondary phase particles. The lighter bands in the optical image reflect a higher density of secondary phase particles, and the darker bands reflect a lower one. Since the visual appearance of the bands depends on the instrumentation used, it is more appropriate to differentiate the bands as “particle-rich” and “particle-poor”, terminology consistent with that introduced by Sutton et al. who observed the same phenomenon in FSW 2024-T351 rolled sheet [10]. The scanning electron images in Fig. 4 taken near the nugget/TMAZ interface even more clearly demonstrate the non-uniform distribution of the secondary phase particles in the banded microstructure. Fig. 4a clearly shows the interleaving of particle-rich and particle-poor layers that develop during FSW. Even more striking, however, is Fig. 4b, which shows a very distinct boundary between a particle-rich region of the weld nugget (right) and the particledeficient TMAZ (left). Some researchers have concluded that the weld nugget structure is actually an interleaving of layers of fine-equiaxed, recrystallized grains with coarse recrystallized grains [17,18]. During this investigation, the grains within the nugget showed little deformation in the flow direction, suggesting that recrystallization did, indeed, occur. The grain size, however, in both the particle-rich and particle-poor bands was equivalent, approximately 10 μm to 20 μm [12]. Fig. 5a is a TEM micrograph of a foil taken from a region near the center of the weld nugget. The secondary phase particles of the particle-rich bands are clearly evident, and chemical analysis by EDS of these phases indicates a strong tin (Sn) content. The particle rich band is primarily composed of the Mg2Si and Mg2Sn phases [19]. Fig. 5b is of a foil taken from the TMAZ, the region adjacent to the weld nugget. The micrograph reveals a high dislocation density in this plastically deformed area, but neither Sn particles nor any strengthening phases are present. Also, foils excised from within the HAZ and base material do not indicate the presence of any Sn-containing phases.
Since tin is only present on the surface of the extrusions as plating, the unique occurrence of tin in the weld nugget is extremely significant. Material comprising the weld nugget, therefore, must originate from the extrusion surface since this is the only viable source of tin. Similarly, the material within the TMAZ and HAZ zones must originate from within the extrusion thickness since TEM demonstrated the absence of tin from these regions. The tin-rich surface and the tindeficient thickness effectively act as tracers of material flow during welding and the evolution of the weld nugget. The TEM results demonstrate that the particle-rich bands containing tin must be comprised of surface material that has flowed into the region that ultimately transforms into the weld nugget. Similarly, since the particle-poor bands do not contain discernable amounts of tin, the material comprising this structure must originate from within the thickness of the extrusions. The banded microstructure of the weld nugget, therefore, is composed of alternating layers of surface material rich in Sn-containing particles and plasticized material already present in the extrusion thickness, i.e. insitu material. Fig. 3 clearly highlights the banded structure of the weld nugget, but it also reveals a ligament, or “tail”, of the weld on the retreating side extending from the primary nugget body to the extrusion surface. The nugget and tail morphology suggests that surface material is introduced from the retreating side into the plasticized region surrounding the FSW pin. As indicated in Fig. 3, the average distance from the tail/ surface intersection to the weld centerline, i.e. the tool center, is 9.0 mm.
Fig. 4 – BSE (a) and SE (b) SEM images near friction stir nugget/ TMAZ interface.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1209
Fig. 5 – TEM micrograph of: a) weld nugget center revealing Sn-rich particles (with EDS spectrum), b) TMAZ revealing no Sn.
These observations are consistent with those made by Colligan in his study of material flow during the friction stir welding of 6061-T6 using steel shot tracers and a “stop action” technique [20]. In examining the flow of material around the FSW keyhole, Colligan noted that below the weld surface, where the steel shot was chaotically distributed, the steel shot was deposited as a continuous line behind the welding pin, akin to the particle-rich bands observed this study. Colligan concluded that surface material was extruded from the retreating side of the pin and deposited in the wake of the tool, in agreement with the tail/nugget morphology of this investigation. Guerra et al. in their study of FSW 6061 with a faying surface tracer and frozen pin technique similarly concluded that material from the front of the retreating side of the pin extrudes between deformed surface material
rotating with the tool and parent material into the area behind the pin [21]. The model of FSW as an extrusion process is further substantiated by Reynolds who used a marker insert technique to study material flow in the welding of 2195-T8 [22]. Reynolds concluded that FSW is essentially an in-situ extrusion process where the tool shoulder, pin, backing plate and base metal effectively form an “extrusion chamber”. Reynolds, however, did not specifically observe extrusion of surface material from the retreating side of the weld. Rather, the tracers in this study revealed a downward motion of surface material on the advancing side coupled with the vertical rise of material from the bottom surface of the retreating side within the pin diameter. Fig. 6 shows a macroscopic plan view of the friction stir welded extrusions. Highlighted in the figure is the flash of
1210
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
Fig. 7 – Schematic representation of plasticized region during friction stir welding. Fig. 6 – Plan view of friction stir welds showing material flash on retreating side.
material consistently present along the length of the weld on the retreating side. The average distance of the flash from the weld centerline is 6.8 mm, a distance that corresponds with the location of the tail/surface intersection of the nugget profile. The presence of the flash and nugget tail on the retreating side identifies this location as the potential source of surface material within the weld nugget. Based upon these results, a model of material flow is proposed for which surface material on the retreating side is extruded into the plasticized zone at a specific radial distance from the tool center, re. The extruding surface material interacts with the in-situ material to form the banded structure of the weld nugget. This model is developed in the proceeding sections.
4.
Proposed Model of Material Flow
4.1.
Conditions for Extrusion
Consider the cross-sectional profile of the FSW process for a typical butt weld configuration. As the tool and pin rotate, the flow stress in a region surrounding the weld joint is exceeded, facilitating solid-state material flow. The shoulder of the rotating tool plasticizes a thin, cylindrical volume of material on the surface, and the pin plasticizes a volume of material within the joint thickness. The net effect is a flow-capable region with a cross-sectional shape resembling that of the inverted bell shown in Fig. 7. On the retreating side of the weld, the rotation of the tool opposes the traversing weld direction. Due to the frictional forces with the tool, the shoulder displaces surface material in both the direction of rotation and in the weld direction. On the retreating side, therefore, surface material rotating with the tool shoulder is forced to flow under material moving in the weld direction as depicted in Fig. 8. In essence, the FSW tool shoulder extrudes a volume of surface material from the retreating side into the plasticized region of the joint. The extrusion process does not occur at
every point along tool radius of the retreating side, but occurs at the radius where the velocity of material following tool rotation is equal and opposite to the velocity of material following the tool traverse. Heurtier et al. identified and modeled three types of motion during friction stir welding, circumventing, torsional and vortex [23]. The circumventing and torsional motions are associated with the flow of surface material, and vortex motion is associated with the flow of thickness material due to the action of the tool pin. Circumventing motion, therefore, is the motion of surface material around the tool shoulder, and torsional motion is the rotational motion of surface material within the interaction layer under the tool shoulder. Heurtier et al. demonstrated that the circumventing velocity, ucir, in the weld direction is given by: cos2h ucir ¼ vw 1 R2 2 r
ð1Þ
where vw is the traversing weld velocity, R is the radius of the tool shoulder, r is the radial distance from the tool center and θ is the radial angle relative to the weld centerline. The torsional velocity along the weld direction, utor, is given by: z utor ¼ r vtor sinh t
ð2Þ
where vtor is the rotational velocity of the material under the tool shoulder, z is the depth into the material, t is the
Fig. 8 – Material flow pattern on retreating side of the weld.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1211
thickness of the interaction layer and all other terms have their previous meaning. Extrusion into the weld nugget then occurs at the location, re, on the retreating side where ucir is equal to utor, such that: cos2h z ¼ re vtor sinh vw 1 R2 2 re t
ð3Þ
Assuming that extrusion occurs at the depth of the interaction layer (z = t) and at a radial angle of − 90° on the retreating side relative to the weld centerline, then Eq. (3) reduces to: re ¼
vw R2 1þ 2 vtor re
ð4Þ
vtor depends on the angular velocity of the tool shoulder, ω, but is certainly lower as the material flow will naturally lag behind the shoulder rotation due to the inherent viscosity. Introducing a scaling factor, β, to account for the lag, the relationship between vtor and ω at the extrusion distance becomes: vtor ¼ b x re
ð5Þ
Combining Eqs. (4) and (5) yields the following expression for the scaling factor:
Fig. 10 – Extrusion ring in the interaction layer with the tool shoulder.
during a single rotation of the tool; therefore, the traverse distance is given by the expression: d¼
2p vw x
ð7Þ
Based on the welding parameters utilized in this investigation (R = 10.5 mm, vw = 5 mm/s, ω = 94.2 s− 1) and setting re to the experimentally measured value of 6.8 mm for the average distance of the flash from the weld centerline, Eq. (6) yields a scaling factor of 0.004.
where all terms have their previous meaning. Surface material from the retreating side is then extruded during each tool revolution through the area shown in Fig. 9. The extrusion area that is swept per tool revolution can be effectively considered the die opening into the plasticized region of the joint thickness. If it is further assumed that opening is rectangular, then the die opening area, Adie, is given by:
4.2.
Adie ¼
b¼
vw R2 1 þ x r2e r2e
ð6Þ
Development of the Extrusion Column
In order to determine the volume of material extruded into the plasticized region, first consider the amount of tool advance in the weld direction per revolution as depicted in Fig. 9. As the tool rotates and traverses in the weld direction, a point initially located at A will displace a distance, d, to point B
2p vw w x
ð8Þ
where w is the width of the die opening. As previously mentioned, the tool shoulder plasticizes a thin, cylindrical volume of material at the weld surface, and an interaction layer of thickness, t, is formed. The material extruded on the retreating side, therefore, originates from the interaction layer and is defined by the volume of material contained within a ring centered on the tool at the extrusion distance, re, as illustrated in Fig. 10. If the width of the extrusion ring is equal to the width of the die opening, w, then the volume of the ring per revolution, Vring, is given by: Vring ¼ p ðre þ wÞ2 t p r2e t
ð9Þ
Simplifying the above equation and removing all second order terms in w yields: Vring ¼ 2p re wt
Fig. 9 – Tool advance and extrusion area per revolution.
ð10Þ
The material extruded into the plasticized region has a rectangular cross-section and a length, l, akin to the production of bar stock during conventional extrusion processes. Assuming that the entire volume of the material ring is extruded into the plasticized region for each revolution of the tool, then the length of the extruded column is found by
1212
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
equating Eq. (10) with the volume of the extrusion, itself, such that: 2p re wt ¼ Adie l
ð11Þ
Substituting Eq. (8) into Eq. (11) and solving for l yields the following expression for the extrusion length: l¼
re tx vw
ð12Þ
Surface material is extruded into the plasticized region under the extrusion force applied by the FSW tool; however, the extrusion is resisted by the drag force of the plasticized material already present in the area. Under the action of these two forces, the extruded column of surface material buckles as represented in Fig. 11. As the tool continues to rotate and traverse, more material is extruded into the region, forcing the buckled, extruded columns further into the plasticized zone and introducing new material into the region. Ultimately, a quasi steady-state is reached such that the region contains bands of extruded surface material interleaved with bands of in-situ thickness material, as observed in Fig. 3. Experimentally determining the amount of material extruded into the plasticized region begins by defining the thickness of the interaction layer, t, as the depth (or thickness) of the nugget tail at the radial extrusion distance. With this definition, the average thickness of the interaction layer for these welding conditions is 0.1 mm. Utilizing this value to evaluate Eq. (12) and using the β-factor of 0.004, the length of the material extruded per tool revolution then becomes 12.6 mm. The microstructure shown in Fig. 3, however, represents quasi steady-state conditions. Assuming that steady-state is achieved within one second, i.e. fifteen tool revolutions, then the length of material extruded per second into the plasticized region is 189 mm. This value predicted by the proposed model can be verified against the experimental data by determining the area fraction occupied by the buckled columns within the weld nugget. The cross-sectional area of the weld nugget in Fig. 3 was estimated as 45 mm2 and that of the buckled, extruded
Fig. 12 – Distribution of particles due to centrifugal force of tool rotation.
material as 35 mm2. The average width of an extruded column within nugget tail is 0.20 mm (a value near the magnitude of the tool advance per revolution, 0.33 mm); therefore, the length of extruded material present in the weld nugget is approximately 175 mm. The experimentally determined value is in good agreement with that predicted by the model (8% difference). Discrepancy between the two values centers on the establishment of steady-state conditions. Though the assumption that quasi steady-state is achieved after one second is reasonable, it is, however, somewhat arbitrary. The model's prediction of the extrusion length is sensitive to the input value of the time required to reach steady-state. For example, if steady-state conditions are established within 0.9 seconds, then the model yields an extrusion length of 170 mm. In the same way, it should be noted that the experimentally determined value of the extrusion length is sensitive to the average width of the extrusion column, which is not easily or accurately measured from the micrographs. Though examination of the weld nugget and the development of the extrusion column validate the proposed model under specific conditions and measurements, it also underscores the difficulty in defining steady-state conditions during FSW and in utilizing average properties to characterize such a complex structure.
4.3.
Fig. 11 – Buckling of extruded surface material in plasticized zone.
Development of the Non-Uniform Particle Distribution
The microstructures of the alloys of interest contain numerous secondary phase particles that are generally coarse and have particle diameters in excess of 5 μm. Within the interaction layer between the tool shoulder and the weld surface, particles will experience a centrifugal force due to the tool rotation that directs them toward the edge of the tool as depicted in Fig. 12. The centrifugal force, however, is resisted by the drag force within the plasticized material. A particle under the action of a centrifugal force, Fcent, will motion until the opposing force, Fdrag, is equal in magnitude. As a result of the tool rotation, larger, heavier particles will be forced to greater distances/radii from the weld joint than smaller, lighter particles. The relationship between the particle radius,
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
1213
Fig. 15 – Build-up of particle rich material at the extrusion location.
Fig. 13 – Microstructure around FSW pin revealing particle banding ~1 mm below surface.
a, and the radial distance from the tool center, r, can be developed by equating the two forces acting on a particle: Fcent ¼ Fdrag
ð13Þ
mv2 1 ¼ qAl v2 ACd 2 r
ð14Þ
Eq. (14) assumes that the plasticized interaction layer is adequately modeled as a fluid exerting drag on a mass in motion; therefore, m is the particle mass, ρAl is the density of aluminum, v is the particle velocity, A is the particle crosssectional area and Cd is the drag coefficient. If it is further assumed that each particle is spherical in shape and has a uniform density of ρpart, then Eq. (14) then reduces to the following expression: r¼
8 qpart 1 a 3 qAl Cd
ð15Þ
Since all terms other than r and a are constant, Eq. (15) can be rewritten as: r ¼ aa
ð16Þ
where α is a constant, and the linear relationship between the size of a particle and the radial distance to which the particle will travel during tool rotation is revealed. Thus, at the
extrusion distance, the surface material will have a higher particle density than both the material located close to the tool center and the plasticized material in the extrusion thickness. Material flowing into the weld nugget from the retreating side, therefore, will have a higher particle density than the in-situ material, creating the non-uniform particle distribution within the nugget. Fig. 13 is an optical micrograph taken approximately 1 mm below the weld surface around the key hole. The characteristic banded microstructure is evident on the surface around the hole, but there is no clear data to support a predictable particle distribution on the weld surface as indicated by Eq. (16). These Sn-rich bands on the surface arise from the plating and do indicate that the tool rotation accumulates Sn-containing particles that then extrude into the nugget zone, forming the characteristic interleaved layers of the weld nugget. The distribution of particles on the weld surface also indicates that the material extruded into the plasticized region at the extrusion distance, re, will have a higher particle density than the material already present in the plasticized zone. As the particle-rich extrusions buckle and continue to press into the region, the steady-state microstructure will contain high particle density bands interleaved with low particle density bands as schematically shown in Fig. 14. In addition, as material enters the plasticized zone, the extrusions will experience a frictional force between the boundary of plasticized material and the “unaffected” material, similar to the interaction between an extrusion billet and the press container during conventional extrusion processes, as represented in Fig. 15 and as experimentally observed in Fig. 3 in the tail section of the weld nugget. The frictional forces with the “unaffected” material cause a reverse material flow, resulting in a build-up of surface material within the plasticized zone at the extrusion distance. Since the extrusion is rich in secondary phase particles, the build-up is primarily characterized by a high particle density.
5.
Fig. 14 – Extrusion of high particle density material in plasticized zone.
Conclusions
Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld configuration. The weld nugget exhibited a banded microstructure consisting of alternating layers of material “rich” in secondary phase particles and material “poor” in the particles. Transmission electron microscopy revealed that the particle-rich bands contained a
1214
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 0 6–1 2 1 4
strong presence of tin, while TEM foils taken from the TMAZ, HAZ and base material thickness did not indicate any Sncontaining phases. Since tin is only present on the surface of the extrusions, the particle-rich bands represent surface material that has flowed into the nugget region. The particle-poor bands with no tin, therefore, must be material that originates from within the thickness of the extrusions. A model of material flow during friction stir welding was proposed. Under this model, the weld nugget forms as surface material is extruded from the retreating side into the region of plasticized material around the FSW pin and under the tool shoulder. As the surface material extrudes into the region, the extrusion column buckles under the influence of the extrusion force driving the material into the area and the drag force of the in-situ material resisting the entry. As a result, a banded microstructure of interleaved surface material and in-situ material develops. The model successfully depicts many of the weld characteristics experimentally observed; however, the efficacy of the model is currently limited to specific conditions and assumptions regarding steady-state material flow.
Acknowledgements The authors would like to specifically thank the Philip and Elaina Hampton Fund for Faculty International Initiatives and the Polish Ministry of Science and Higher Education (Grant No. N 507 094 32/2648) for making this research collaboration a reality.
REFERENCES [1] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, G.B. Patent Application No. 9125978.8 (December 1991). [2] Dawes C, Thomas W. TWI Bulletin November/December 1995;6:124. [3] Sutton MA, Yang B, Reynolds AP, Yan J. Mat Sci Eng A 2004;364:66.
[4] Dracup BJ, Arbegast WJ. Proceedings of the 1999 SAE Aerospace Automated Fastening Conference & Exposition, Memphis, TN; October 5–7, 1999. [5] von Strombeck A, dos Santos JF, Torster F, Laureano P, Kocak M. Proceedings of the First International Symposium on Friction Stir Welding. CA, USA: Thousand Oaks; June 14–16, 1999. [6] Hori H, Tanikawa H, Seo N, Namba K. Proceedings of the 6th International Symposium on Friction Stir Welding. Canada: Montreal; October 10 – 13, 2007. [7] Mishra RS, Ma ZY. Mater Sci Eng R Rep 2005;50:1. [8] Arbegast WJ. In: Jin Z, Beaudoin A, Bieler TA, Radhakrishnan B, editors. Hot deformation of aluminum alloys III. Warrendale, PA, USA: TMS; 2003. p. 313. [9] Krishnan KN. Mat Sci Eng A 2002;327:246. [10] Sutton MA, Yang B, Reynolds AP, Taylor R. Mater Sci Eng A 2002;323:160. [11] Seidel TU, Reynolds AP. Sci Technol Weld Join 2003;3(8):175. [12] Hamilton BC, Dymek S, Blicharski M. Arch Metall Mater 2007;52:66. [13] Hamilton C, Dymek S, Blicharski M. Proceedings of the 2007 TMS Annual meeting and exposition, Orlando, FL; February 25 – March 1, 2007. [14] Attallah MM, Davis CL, Strangwood M. proceedings of the 6th International Symposium on Friction Stir Welding. Canada: Montreal; October 10 – 13, 2007. (on CD-ROM). [15] Thomas WM, Nicholas ED, Smith SD. In: Das SK, Kaufman JG, Lienert TJ, editors. Aluminum 2001 – Proceedings of the TMS 2001 Aluminum automotive and joining sessions. TMS; 2001. p. 213. [16] Thomas WM, Johnson KI, Wiesner CS. Adv Eng Mater 2003;5:485. [17] Mahoney MW, Rhodes CG, Flintoff JG, Spurling RA, Bingel WH. Metall Mater Trans A 1998;29:1955. [18] Sato YS, Park SHC, Kokawa H. Metall Mater Trans A 2001;32:3023. [19] Jung IH, Kang DH, Park WJ, Kim NJ, Ahn S. Calphad 2007;31:192. [20] Colligan K. Weld J 1999;78(7):229S. [21] Guerra A, Schmidt C, McClure JC, Murr LE, Nunes AC. Mat Char 2002;49(2):95. [22] Reynolds AP. Sci Technol Weld Join 2000;5(2):120. [23] Heurtier P, Jones MJ, Desrayaud C, Driver JH, Montheillet F, Allehaux D. J Mater Process Technol 2006;171:348.